1. Field of the Invention
The present invention relates to an ultrasonic imaging apparatus and ultrasonic velocity optimization method which can automatically optimize a sound velocity in ultrasonic imaging used for image diagnosis, nondestructive inspection, or the like.
2. Description of the Related Art
Examples of an ultrasonic imaging apparatus which performs imaging by using ultrasonic waves include an ultrasonic inspection apparatus for nondestructively inspecting abnormality in a structure and an ultrasonic diagnosis apparatus which transmits ultrasonic waves to a subject (patient) and acquires tomograms associated with a diagnosis region on the basis of the reflected waves. For example, an ultrasonic diagnosis apparatus can display, in real time, how a heart beats or a fetus moves, with simple operation of bringing an ultrasonic probe into contact with the body surface. In addition, this apparatus offers a high level of safety, and hence can be repeatedly used for examination. Furthermore, the system size is smaller than those of other diagnosis apparatuses such as X-ray, CT, and MRI apparatuses. Therefore, this apparatus allows easy examination upon being moved to a bed side. That is, the apparatus is a convenient diagnosis technique. Ultrasonic diagnosis apparatuses used in such ultrasonic diagnosis vary depending on the types of functions which they have. Some compact apparatuses which can be carried with one hand have been developed. Ultrasonic diagnosis is free from the influence of radiation exposure such as X-ray exposure, and hence can be used in obstetric treatment, treatment at home, and the like.
An ultrasonic imaging apparatus typified by such an ultrasonic diagnosis apparatus uses a method of converging transmission and reception beams to improve the azimuth resolution of an image. Electronic scanning type array transducers, in particular, use an electronic convergence method based on delay time control for transmission/reception signals of each channel. A problem in this electronic convergence method is that a beam diverges at a place (depth) apart from a convergence point, and the azimuth resolution decreases.
For this problem, a conventional ultrasonic imaging apparatus uses a technique called a dynamic convergence method. This technique performs delay time control to continuously move a convergence point in the depth direction with a lapse of time at the time of reception. This technique allows to always acquire a reception ultrasonic beam from a converged area.
FIG. 13 is a view showing the positional relationship between each ultrasonic transducer of an ultrasonic probe and a focal point P in a subject to be examined. As shown in FIG. 13, letting X be the coordinates of the focal point P in the depth direction, and Yi be the coordinates of an ultrasonic transducer Ti in the array direction from the aperture center (origin O) of the ultrasonic probe, a delay time Δti from the time when the wavefront of a reflected sound wave reaches the aperture center to the time when the wavefront reaches the ultrasonic transducer Ti is given byΔti=[(X2+Yi2)1/2−X]/C where C is a sound velocity.
In this calculation, if the sound velocity used for the calculation is equal to the actual sound velocity of propagation in the subject, as shown in FIG. 14A, desired positions Fn−1, Fn, and Fn+1 can be made to coincide with the beam convergence point, thereby acquiring a high-resolution ultrasonic image.
A conventional ultrasonic diagnosis apparatus, however, calculates the delay time Δti by using a preset velocity (representative velocity) v representing a visualization target slice regardless of the position of the slice and the components of a progation medium, and sets the calculated time. The actual sound velocity of propagation in the subject does not always coincide with the representative velocity v. If, for example, the representative sound velocity used for calculation is lower than the actual sound velocity of propagation in the subject, as shown in FIG. 14B, a beam convergence point is located before the desired positions Fn−1, Fn, and Fn+1, resulting in a low resolution as compared with the case shown in FIG. 14A.
It has recently been reported that C=1560 cm/s in the muscle, and C=1480 cm/s in the fat. In addition, the sound velocity varies in individuals. The difference between the representative sound velocity v and the actual sound velocity of propagation C causes the difference between the assumed position of a convergence point and the actual position of the convergence point, resulting in image degradation.
As techniques for solving the difference between the assumed position of a convergence point and the actual position of the convergence point in a conventional ultrasonic diagnosis apparatus, techniques such as phase correction techniques based on a reflection method and a cross-correlation method are available. These techniques, however, require the presence of a reflector such as a calculus or a boundary wall, and has limitations such as the necessity of the presence of a reflector as a point. Even if, therefore, these techniques are used, it is impossible to acquire good images as a whole.